A multistage printing machine of this character has been illustrated, for example, in U.S. Pat. No. 3,974,766. In that system the necessary correlation between the several printing units or stages is achieved with the aid of mechanical couplings interconnecting the drive shafts of their rotary screens so as to transmit a command from a control circuit down the cascade with the necessary delay depending upon the transport speed of the substrate and the angular positions of the pattern-forming sections of the several screens. The system also includes means for raising and lowering each screen with reference to the substrate, with the screen contacting the substrate only during a working phase of a printing cycle and being lifted off during an idling phase. In the latter phase a supplemental rotation is imparted to the screen for the purpose of returning it to its starting position before the beginning of the next cycle.
The system of U.S. Pat. No. 3,974,766 is particularly designed to imprint a substrate divided into relatively short sections which advance beneath the screens in a strictly synchronous manner, i.e, with their transport speed equaling the peripheral screen velocity. Such synchronism is not always desirable and should be avoided, for example, with certain pile fabrics having stiff upstanding tufts which may damage the screens unless a positive or negative speed differential causes their deflection onto the supporting fabric. Such a speed differential can also be used to modify the length of the printed pattern, e.g. for the purpose of compensating longitudinal shrinkages of the substrate.
Conventional mechanical transmissions interlinking the screens of such printing units are difficult to adjust for the purpose of allowing a selected slip to occur between the screens and the substrate. If the delay in the transmission of operating commands between successive printing units or stages is a function of transport speed, adjustments for a selected slip must also be made in the compensatory rotation imparted to each screen during its idling phase.